1. Field of the Invention
The present invention relates to the generation of chlorine dioxide and more particularly relates to an improved method of chlorine dioxide generation wherein the resultant conversion of chlorite to chlorine dioxide is of an efficiency previously unknown using a two chemical system employing sulfuric acid/sodium chlorite, and in which the precursors can be retained in the reactor for a prolonged time before dilution without loss of chlorine dioxide by reaction with water to form chlorate, as has been reported when hydrochloric acid is used as the proton donor.
2. General Background
Chlorine dioxide is a powerful oxidant and disinfectant. Applications for chlorine dioxide cover a wide spectrum from disinfection of foods and drinking water, treatment of process water, odor control, zebra mussel eradication, Anthrax destruction, disinfection of medical waste, wastewater treatment, and oil- and injection water well stimulation, paper pulp bleaching, and fabric bleaching.
Chlorine dioxide is not available for purchase or may not be readily available for every application in which it might be used. In certain situations, regulatory and economic limitations suggest that the chloring dioxide cannot be shipped, but instead must be generated on site at the time of use. The need for generation has spawned a variety of processes in which a relatively small group of precursors are combined in different ways. These can be broken down into groups depending on the precursor and the method of conversion.
Two generally indispensable precursors around which many chlorine dioxide generation methods are built are sodium chlorate, NaClO3, and sodium chlorite, NaClO2. Sodium chlorate is the less expensive of the two and, as such, has become the precursor of choice for the paper industry, which uses chlorine dioxide daily in tonnage quantities to bleach and delignify paper pulp, as well as for applications such as wet-end biological control on paper machines. Lowering chemical costs justifies the investment in corrosion resistant, operator-controlled titanium machinery suit to carry-out acidic chlorate conversion. There is a commercially available small-scale, three-chemical method for chlorate conversion U.S. Pat. No. 6,790,427, which is hereby incorporated by reference in its entirety. The method teaches the combination of concentrated sulfuric acid, and a proprietary mix of sodium chlorate and hydrogen peroxide to convert the chlorate to chlorine dioxide.
Other known generation methods employ sodium chlorite, despite its higher cost, because of the relative ease of conversion. Conversion methods can be categorized as one chemical, two chemical, and three chemical, each of which offers a specific advantage. One chemical method includes electrolytic oxidation of chlorite anion, and exposure to ultraviolet light. Current electrolytic methods can generate hundreds of pounds of chlorine dioxide per day, whereas ultraviolet methods are useful in cases where a few pounds per day are adequate.
In view of the shortcomings of the prior art, it would be desirable to have an improved method of generating chlorine dioxide using two precursor chemicals that may result in high conversion rates, be able to be carried-out in situ, when necessary, and be conducted in a scaleable manner, to meet the needs of large users of ClO2, such as paper mills, and small users such as private water treatment facilities.
Further, many acid-chlorite methods are known. Such methods that employ hydrochloric acid are known to yield no greater than a theoretical 80% conversion rate of the chlorite used into chlorine dioxide, with practical yields closer to 70%. Diluted (9-15%, by weight) hydrochloric acid is commonly used as the acid in the generation of chlorine dioxide from chlorite. Concentrated sulfuric acid cannot be combined directly with chlorite, as it reacts too violently and generates a significant amount of heat, which lends to volatilization of produced chlorine dioxide and possible damage to plastic generation equipment.
It would, therefore, be desirable to have a method which employs ‘dilute sulfuric acid’ in the generation of chlorine dioxide and results in higher conversion efficiencies than were previously known for this chemistry with little or no conversion of generated chlorine dioxide to chlorate even with prolonged residence time in the reactor. This increase in conversion efficiency would result in obvious economic advantages over previously known methods.